The present invention relates to a baccatin derivative and a process for producing the same, and in particular to a baccatin derivative of the general formula (I) comprising xcex2-ketoester bound to a baccatin whose hydroxyl groups at the 7- and 10-positions are protected, as well as a process for producing the same. This material is useful for preparing taxoid compounds such as paclitaxel.
Paclitaxel (trade name: Taxol) is one kind of anticancer agent taken from a Taxus brevifolia (yew tree) and it is known to be effective particularly against breast cancer and lung cancer. However, the amount of paclitaxel taken from the Taxus brevifolia is very small, and the problem of destruction of forests is caused by stripping the bark from the tree.
On the other hand, 10-deacetylbaccatin III can be taken again because this compound is obtained from leaves of the tree and it is useful as a precursor of paclitaxel or its derivative docetaxel (trade name: Taxotere).
For synthesis of the taxoid compounds, semi-synthetic methods are known, and the following methods have been reported: (a) a method by using xcex2-lactam (European Patent No. 0400971), (b) a method by using an oxazoline compound (International Patent Kokai No. 504444/1995), (c) a method by using a thioester compound (International Patent Kokai No. 505360/1998), and (d) a method by using cinnamic acid (Tetrahedron, Vol. 42, p. 4451 (1986)). These methods are related to the esterification for binding a carboxylic acid compound to an unprotected hydroxyl group at the 13-position in baccatin, or to the esterification using an activated carboxylic acid (thioester).
In general, the preparation of ester compounds can be accomplished by a method of binding a carboxylic acid compound to an alcohol compound with a condensation agent such as dicyclohexylcarbodiimide or diisopropylcarbodiimide in the presence of abase such as pyridine or 4-dimethylaminopyridine; by a method of using an acid anhydride/acid halide; or by transesterification using an acid catalyst etc. For example, the transesterification using an ester compound and an alcohol compound is known as a general method as described in publications such as Chemical Review, Vol. 93, p. 1449 (1993) and Journal of Organic Chemistry, Vol. 50, p. 3618 (1985).
Up to now, the reaction of introducing a side-chain moiety onto a hydroxyl group at the 13-position is limited to the method of binding an carboxylic acid and an activated carboxylic acid (thioester) to the hydroxyl group as described above, and there has been no report on a method of introducing an ester compound as a precursor of a side-chain moiety into the hydroxyl group at the 13-position by transesterification. By introducing an ester compound as a precursor of a side-chain moiety, a compound having a different functional group to that of the conventional side-chain moiety can be easily prepared, and the possibility of obtaining a compound having a different physiological activity than ever before is suggested. In general, the transesterification is conducted in the presence of an acid catalyst such as sulfuric acid or p-toluenesulfonic acid, an amine base such as 4-dimethylaminopyridine or 1,8-diazabicyclo[5,4,0]undecene, or titanium tetraalkoxide etc., but even the transesterification where the reaction proceeds between alcohol and ester is also reported in e.g. Journal of the American Chemical Society, p. 4195 (1951).
In view of the circumstances described above, the inventors have extensively studied a method of introducing a side-chain moiety precursor by transesterification and attempted to develop a baccatin derivative having xcex2-ketoester bound via an ester linkage to a hydroxyl group at the 13-position in baccatin and a process for producing the same.
As a result, the inventors have found that when xcex2-ketoester is allowed to react with a baccatin in the presence of either a tin compound or an amine base preferably under reduced pressure, the xcex2-ketoester is bound via an ester linkage to the baccatin by transesterification, and the present invention was thereby completed.
Further, the inventors have extensively studied a method of introducing a side-chain moiety precursor by transesterification in the absence of a catalyst and attempted to develop a baccatin derivative having a xcex2-ketoester bound via an ester linkage to a hydroxyl group at the 13-position in baccatin, as well as a process for producing the same.
The present invention has the following aspects and embodiments:
(1) The first aspect of the present invention relates to a baccatin derivative represented by the general formula (I): 
(wherein R1 and R2 simultaneously or independently represent a hydroxyl-protecting group, R3 represents any one group selected from the group of an unsubstituted or substituted phenyl group, an unsubstituted or substituted furyl group, an unsubstituted or substituted pyridinyl group, an alkyl group, a hydroxyalkyl group, a halogenated alkyl group, a cyclic alkyl group and a thienyl group, R5 represents a hydrogen atom or an alkyl group, Bz represents abenzoyl group, and Ac represents an acetyl group).
(2) The second aspect of the present invention relates to a process for producing a baccatin derivative represented by the general formula (I) described above, which comprises allowing a baccatin to react with a xcex2-ketoester in the presence of a tin compound or an amine base, wherein the baccatin is represented by the general formula (II): 
(wherein R1 and R2 simultaneously or independently represent a hydroxyl-protecting group, Bz represents a benzoyl group, and Ac represents an acetyl group).
(3) As the third aspect of the present invention, in (2) above, the reaction is conducted under reduced pressure.
(4) The forth aspect of the present invention also relates to a process for producing a baccatin derivative represented by the general formula (I) above, which comprises allowing a baccatin represented by the general formula (II) to react with a xcex2-ketoester in the absence of a catalyst: 
(wherein Rand R2 simultaneously or independently represent a hydroxyl-protecting group, Bz represents a benzoyl group, and Ac represents an acetyl group).
(5) As the fifth aspect of the present invention, in (4) above, the reaction is conducted under reduced pressure.
(6) The sixth aspect of the present invention further relates to a baccatin derivative represented by the general formula (III): 
(wherein R1 and R2 simultaneously or independently represent a hydroxyl-protecting group, n is an integer of 1 to 5, Bz represents a benzoyl group, and Ac represents an acetyl group)
(7) The seventh aspect of the present invention relates to use of a baccatin derivative represented by the general formula (I) described in (1) above or a baccatin derivative represented by the general formula (III) described in (6) above for producing taxoid compounds such as paclitaxel.
Hereinafter, the present invention is described in detail.
The baccatin used in the present invention can be 10-deacetylbaccatin III extracted from yew trees, a compound analogous thereto, or a compound obtained by synthesis from a low-molecular compound. In particular, 10-deacetylbaccatin III is suitable for efficiently achieving the present invention.
10-Deacetylbaccatin III used in the present invention, to which a protecting group was introduced, is represented by the general formula (II) above.
The hydroxyl-protecting group in the above formula includes protecting groups described in e.g. xe2x80x9cNew Course of Experimental Chemistry, 14, Organic Synthesis V, Chapter 11-1, compiled by the Chemical Society of Japanxe2x80x9d. Specific protecting groups include triethylsilyl group, benzyloxycarbonyl group, acetyl group, allyloxycarbonyl group etc.
Then the xcex2-ketoester used in the present invention is represented by any of the following formulae: 
(wherein R3 is a group selected from an unsubstituted or substituted phenyl group, an unsubstituted or substituted furyl group, an unsubstituted or substituted pyridinyl group, an alkyl group, a hydroxyalkyl group, a halogenated alkyl group, a cyclic alkyl group and a thienyl group, preferably a phenyl group, p-methoxyphenyl group, 2-furyl group, o-trifluoromethyl phenyl group, m-fluorophenyl group and cyclohexyl group. R4 is a nucleus of alcohol for forming an ester with a carboxyl group and includes a methyl group, ethyl group, isopropyl group and allyl group); 
(wherein, R3 is a group selected from an unsubstituted or substituted phenyl group, an unsubstituted or substituted furyl group, an unsubstituted or substituted pyridinyl group, analkyl group, ahydroxyalkyl group, ahalogenatedalkyl group, a cyclic alkyl group and a thienyl group. Specifically, as R3 may be mentioned a phenyl group, p-methoxyphenyl group, 2-furyl group, o-trifluoromethylphenyl group, m-fluorophenyl group or cyclohexyl group. R4 is a nucleus of alcohol forming an ester with a carboxyl group and includes a methyl group, ethyl group, isopropyl group and allyl group. R5 is a hydrogen atom or an alkyl group, examples of the alkyl group include a methyl group, ethyl group etc.); 
(wherein, R4 is a nucleus of alcohol forming an ester with a carboxyl group, and specific examples include methyl group, ethyl group, isopropyl group and allyl group. Further, n is an integer of 1 to 5).
The xcex2-ketoester may be used a commercial product, or it can be prepared by reacting an acid chloride with methyl acetoacetate. The acid chloride in this case is obtained by a general reaction of carboxylic acid with oxalyl chloride. Specific examples of the carboxylic acid include methoxybenzoic acid, monofluorobenzoic acid, hydroxybenzoic acid, trifluoromethylbenzoic acid.
The xcex2-ketoester used in the present invention includes e.g. methyl p-methoxybenzoylacetate, methyl o-trifluoromethylbenzoylacetate, methyl m-trifluoromethylbenzoylacetate, methyl p-trifluoromethylbenzoylacetate, methyl o-fluorobenzoylacetate, methyl m-fluorobenzoylacetate, methyl 2-furanoylacetate, methyl cyclohexanoylacetate, methyl 2-oxocyclopentylacetate, methyl 2-methylbenzoylacetate etc.
The reaction of baccatin with xcex2-ketoester can be conducted by using excess amount of xcex2-ketoester (5 to 30 equivalents) without adding other solvent. However, the reaction can also be conducted in the presence of other solvent optionally, and such solvent includes solvent having high boiling point such as diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and cymene.
The process of the second aspect of the present invention is conducted in the presence of a tin compound or an amine base. Even if a titanium compound which is generally used in transesterification, such as tetraisopropyl titanate other than the compound described above, the desired compound can be obtained even though the yield is low and a large amount of byproducts are produced. The tin compound includes 1-chloro-3-hydroxy-tetrabutyldistannoxane, 1,3-dichlorotetrabutyldistannoxane etc., and the amine base includes 4-dimethylaminopyridine (DMAP), 4-pyrrolidinopyridine, N,N-dimethylaniline, 1,8-diazabicyclo[5,4,0]-7-undecene (DBU), tri-n-octylamine etc. The reaction may be conducted by adding the tin compound in an amount of 2%, based on baccatin, and the amine base in an amount up to 2 equivalents, based on baccatin.
In addition, the process of the forth aspect of the present invention may also be conducted in the absence of a catalyst used in general transesterification.
Further, the reaction of baccatin with xcex2-ketoester can also be conducted at normal pressure, but this results in a long reaction time, so the reaction is conducted preferably at a reduced pressure of 0.5 to 400 mmHg using a vacuum device such as aspirator and vacuum pump. In particular, preferred conditions are 0.5 to 1 mmHg when the reaction is conducted in the absence of a solvent or 20 to 40 mmHg when a solvent is added. The reaction is conducted at 60 to 120xc2x0 C., preferably 90xc2x0 C., for 1.5 to 24 hours, preferably 2.5 to 5 hours.
An excess amount of xcex2-ketoester is recovered by trapping it in the line for reducing pressure and can be utilized again.
Hereinafter, the present invention is described specifically by reference to a typical example in which the baccatin used is 10-deacetylbaccatin III.
10-Deacetylbaccatin III wherein hydroxyl groups at 7- and 10-positions have been protected can be produced by the following reaction scheme I: 
Triethylsilyl chloride, imidazole and dichloromethane are added to 10-deacetylbaccatin III (Compound (1)), and the mixture is reacted at 0 to 100xc2x0 C., preferably 20xc2x0 C., for 0.5 to 100 hours, preferably 3 hours, to give Compound (2) wherein a hydroxyl group at the 7-position has been protected.
Benzyloxycarbonyl chloride, 4-dimethylaminopyridine and dichloromethane are added to said compound (2), and the mixture is reacted at xe2x88x9220xc2x0 C. to 30xc2x0 C., preferably 0xc2x0 C., for 0.5 to 100 hours, preferably 14 hours, to give Compound (3) wherein a benzyloxycarbonyl group has been introduced at the 10-position.
The compound of the general formula (I) having xcex2-ketoester introduced by transesterification into a hydroxyl group at the 13-position in 10-deacetylbaccatin III having protected hydroxyl groups at the 7- to 10-positions, can be produced by the following reaction scheme II: 
The xcex2-ketoester and, if necessary, a tin compound or an amine base as a catalyst are added to Compound (3), that is, 10-deacetylbaccatin III wherein hydroxyl groups at the 7- and 10-positions have been protected, and the mixture is reacted under reduced pressure at 80 to 120xc2x0 C., preferably 90xc2x0 C., for 1.5 to 24 hours, preferably 5 hours, whereby an ester compound (Compound (4)) is obtained.
Taxoid compounds can be prepared via several steps from the baccatin derivative obtained in the present invention as the starting material.
As the resulting taxoid derivatives besides paclitaxel and docetaxel, it is possible to obtain compounds having a non-phenyl functional group as the side chain at the 3xe2x80x2-position; compounds having a functional group other than a benzoyl group or t-butoxycarbonyl group on an amino group at the 3xe2x80x2-position; and compounds having various acyl groups bound to hydroxyl groups at the 7- and 10-positions, and it can be expected that compounds having antitumor activity, which is different from that of compounds known so far are, obtained.
As described in detail hereinbefore, the present invention provides a baccacin derivative by binding xcex2-ketoester via an ester linkage to a hydroxyl group at the 13-position in baccatin such as 10-deacetylbaccatin III through transesterification therebetween in the absence of a catalyst or in the presence of a tin compound or an amine base, preferably under reduced pressure, as well as a process for producing the same.
Further, the baccatin derivatives of the present invention are useful as starting materials for preparing taxoid compounds such as paclitaxel as an anticancer agent.